1. Field of the Invention
The present invention relates to packaging for optically interactive electronic devices. More particularly, the present invention relates to a package for a solid-state image sensor particularly suitable for high input/output (I/O) count devices wherein an image sensor chip is mounted in flip-chip fashion to a transparent substrate or to a secondary substrate secured to the transparent substrate.
2. State of the Art
Optically interactive electronic devices, for example, charge coupled device (CCD) image sensors or complementary metal-oxide semiconductor (CMOS) image sensors, are typically packaged within a housing for subsequent connection to higher-level packaging such as a larger circuit assembly in the form of a carrier substrate. The housing provides electrical interconnection to the larger circuit assembly, provides protection from the surrounding environment and allows light or other forms of radiation to pass through to sensing circuitry located on the image sensor device. In the prior art, package formation has often been accomplished by placing an image sensor device in the form of a semiconductor chip into the cavity of a plastic or ceramic housing, wire bonding electrical connection points on the semiconductor chip to conductive pads associated with the housing and sealing a window or transparent cover over the cavity. The materials and structure involved with this packaging technique require a fabrication process that can be time consuming and requires several precision assembly steps. Wire bonding, for instance, involves special considerations during package formation due to the fragile nature of bond wires and their associated connection points, and also may call for excessive package depth or thickness in order to accommodate the arched wire bond loops within the package cavity. Further, each assembly step increases the opportunity for contamination or damage to the image sensor device itself, raising defect levels and slowing production time to avoid such damage and contamination. Due to the extremely cost-competitive nature of today""s semiconductor industry, even small improvements in product yield and production time are of value, especially when considered in terms of the high volume of components being manufactured.
In response to the above-noted shortcomings of the existing packaging techniques, various structures using flip-chip mounting of an image sensor chip have been developed in an attempt to simplify the construction of image sensor packages. U.S. Pat. No. 6,144,507 to Hashimoto and U.S. Pat. No. 5,867,368 to Glenn, for example, each disclose an image sensor chip mounted directly to a printed circuit board (PCB). An image sensor chip is mounted in flip-chip fashion over an aperture within the PCB, and a transparent cover is either attached directly to the active surface of the chip or bonded to the side of the PCB opposite that to which the image sensor chip is attached and over the aperture. Although these methods eliminate the difficulties associated with wire bonding and forming a housing for the image sensor chip, the illustrated PCBs are very large with respect to the size of the image sensor chip and the transparent cover. It is unclear from the aforementioned patents whether the PCBs comprise discrete image sensor packages suitable for attachment to a larger circuit assembly, or themselves comprise large circuit assemblies simply having an image sensor chip mounted directly thereto, without the benefit of a package or housing.
Another packaging approach has been to use the transparent cover itself as a foundation for an image sensor package. U.S. Pat. No. 5,786,589 to Segawa et al. uses this approach by adhesively bonding a TAB sheet to a glass substrate and bonding an image sensor chip to the TAB tape with an anisotropically conductive film. This design requires a specialized substrate attachment technique due to the TAB-type connection leads. Moreover, the conductive film risks interference with sensing circuitry on the image sensor chip and requires the formation of dummy leads or dam structures to compensate for this problem. U.S. Pat. No. 6,342,406 to Glenn et al. and U.S. Pat. No. 5,352,852 to Chun each bond an image sensor chip to a transparent substrate by forming conductive traces directly on the substrate. Glenn et al., however, uses a ball mounting arrangement on the first surface of the transparent substrate that requires the formation of vias through the substrate to connect interior and exterior traces formed thereon. The arrangement also requires an aperture be formed within any substrate carrying the package to provide optical access to the active surface of the image sensor chip, as the image sensor active surface faces the attachment side of the package. Chun uses a lead arrangement that requires the package to be mounted within a substrate mounting depression or the addition of outleads which may be susceptible to damage during package handling and may not be suitable for high I/O sensor devices. Chun further requires the use of a frame of insulating tape to space the image sensor chip from the interior of the transparent substrate.
As is evident from the foregoing review of the current state of the art in image sensor packaging, a need exists for an improved image sensor packaging structure that is simple to fabricate, suitable for use with high I/O sensor devices, durable and easily mounted to a larger circuit assembly without the use of special substrate apertures or depressions.
In accordance with the present invention, image sensor packaging having the above-described and other beneficial characteristics and methods for fabrication thereof are provided. An image sensor chip is flip-chip mounted to conductive traces on a first surface of a transparent substrate. The active surface of the image sensor chip is protected from contamination after mounting by depositing a bead of sealant around the periphery of the image sensor chip between the active surface of the image sensor chip and the first surface of the substrate, thus eliminating any need for additional damming structures or spacing frames as used in the prior art. Discrete conductive elements such as solder balls or columns are attached to ends of the conductive traces which form an array pattern, the discrete conductive elements extending transversely from the conductive traces on the first surface to a substantially common plane at a level beyond a back surface of the image sensor chip. The resulting structure comprises a board-over-chip (BOC) package arrangement that provides high I/O connectivity for the image sensor chip and is easily mounted to a carrier substrate such as a printed circuit board (PCB) without requiring the formation of special apertures therein.
In a first exemplary embodiment of the present invention, conductive traces are formed directly on the first surface of the transparent substrate and an image sensor chip is flip-chip mounted to first attachment points of the conductive traces. The conductive traces extend beyond the periphery of the image sensor chip, and discrete conductive elements are joined to or formed on second attachment points of the conductive traces in an array pattern around the image sensor chip.
In a second exemplary embodiment of the present invention, a secondary substrate having the conductive traces formed thereon is secured to the first surface of the transparent substrate. An image sensor chip is flip-chip mounted to first attachment points of the conductive traces. The conductive traces extend beyond the periphery of the image sensor chip and discrete conductive elements are formed in an array pattern as in the first exemplary embodiment.
In a third exemplary embodiment of the present invention, a backing cap is placed over the back surface of the image sensor chip. The side of the backing cap facing toward the image sensor chip carries contacts placed in electrical communication with the conductive traces formed on the transparent substrate or the secondary substrate which are, in turn, in electrical communication with the image sensor chip. Because the backing cap covers the back surface of the image sensor chip, discrete conductive elements for connection to external circuitry may be formed in a full array pattern including the area underneath the image sensor chip.
In a fourth exemplary embodiment of the present invention, the secondary substrate secured to the transparent substrate is in the form of a flex circuit having the conductive traces formed thereon. The flex circuit includes a backing portion that is folded over the back surface of the image sensor chip. The backing portion has discrete conductive elements formed in a full array pattern as in the third exemplary embodiment.
In a further variation to any of the above-described package embodiments, an array of image sensor packages may be simultaneously formed on one large transparent substrate, which is then cut or otherwise divided or singulated to form multiple individual image sensor packages.
Other and further features and advantages of the present invention will be apparent from the following descriptions of the various exemplary embodiments read in conjunction with the accompanying drawings. It will be understood by one of ordinary skill in the art that the following are provided for illustrative and exemplary purposes only, and that numerous combinations of the elements of the various embodiments of the present invention are contemplated as within the scope of the present invention.